Qatar Airways has confirmed that the parcel bomb discovered in the United Arab Emirates was carried on two of its passenger jets.

The announcement by Qatar's flagship carrier came as a senior US administration official expressed fears that cargo bombs will be used again as a bomb plot.

John Brennan, the US counterterrorism chief, said on Sunday there was no intelligence indicating there were any additional parcel bombs from Yemen, although he refused to rule out the possibility.

"I think we have to presume that there might be and, therefore, we have to take these measures. But right now we do not have indications that there are others that are out there," he told NBC's Meet the Press news show.

Brennan, who is the US administration's deputy national security adviser, separately told ABC's This Week that evidence suggested the same person constructed the Yemen parcel bombs and the device worn by Umar Farouk Abdulmutallab, a Nigerian man who attempted to ignite a bomb on board an airliner over the US on December 25 last year.

Qatar Airways statement

Security officials have been on high alert since the UK and the UAE intercepted on Friday two packages destined for synagogues in Chicago.

Qatar Airways said on Sunday that one courier consignment was carried aboard one of its aircraft from Sanaa, the Yemeni capital, to Dubai via Doha International Airport.

The airline also stated that "the explosives discovered were of a sophisticated nature whereby they could not be detected by X-ray screening or trained sniffer dogs", and were only discovered after an intelligence tip-off.

However, Qatar Airways' chief executive, Akbar al-Baker, said the airline's security is as tight as it possibly can be.

"Quite frankly, I don't know what more improvement we can do when there are explosives in the hands of terrorists that cannot be detected by sniffer dogs [or] any available technology today," he said.

"We are doing our best.

"These packages should have been inspected by the courier company. They should have been inspected by the government from where this package was loaded on the airplane.

"It is not the job of the airlines to inspect baggage. It is entirely the job of regulators and governments."

The admission by Qatar Airways comes against a backdrop of fresh revelations about the bomb plot.

Britain's Sunday Telegraph newspaper and The Washington Post reported on Sunday that investigators were focusing on Ibrahim Hassan al-Asiri, an al-Qaeda explosives expert based in Yemen.

The Sunday Telegraphsaid that the 28-year-old Saudi national recruited his younger brother, Abdullah, as a suicide bomber who tried to kill Saudi Arabia's Prince Mohammed bin Nayef in 2008, using pentaerythritol tetranitrate, or PETN.

PETN is the same substance that was packed into the underwear of Abdulmutallab.

Police in Dubai have said in a statement that the package they found, which contained a printer ink cartridge packed with PETN, bore all the hallmarks of al-Qaeda.

Al Jazeera's Dan Nolan, reporting from Dubai, said that PETN, an odourless plastic explosive, "seems to be virtually undetectable" in airport security screenings.

Packages halted

Taking precautionary measures, both UPS and Fedex said they had halted all packages being sent from Yemen to the US while the incident is investigated.

The failed plot has prompted scrutiny of airport security in the UK, where Theresa May, the home secretary, announced a ban on all unaccompanied cargo coming from Yemen into the country.

Security has also increased in the US, where the postal service has temporarily stopped accepting inbound mail originating in Yemen. Germany has banned air freight from Yemen from entering the country.

The detention of a student on suspicion of mailing the packages set off protests at Sanaa University [Reuters]

The developments came as Yemen released a 22-year-old female engineering student arrested in connection with the parcel-bomb plot.

Hanan al-Samawi was detained on Saturday on suspicion of mailing the packages, which were discovered on Friday at UK's East Midlands Airport and at a courier facility in Dubai.

But dozens of students at Sanaa University protested on Sunday against al-Samawi's arrest, saying she was innocent.

"The Sanaa University student union ... believes the girl is innocent and has been wronged," the union's president, Ridhwan Massoud, said.

Ahmed al-Samawi, a relative of the arrested student, also expressed concern about her arrest.

"The Yemeni security has to be careful not to tarnish anyone's reputation," al-Samawi told Al Jazeera.

"I don't think anyone who would like to commit such act, would leave his/her personal ID behind to be taken as evidence to implicate him/her. Quite frankly, I don't understand the whole story from the start."

Yemeni authorities are engaged in a hunt for al-Qaeda fighters, where Anwar al-Awlaki, a US-born Muslim religious leader, is named as being linked to the bomb plot.